Thermal imprint lithography has been recently studied and developed as a low cost alternative technique for patterning features in the surface of a substrate or workpiece, as for example, described in U.S. Pat. Nos. 4,731,155; 5,772,905; 5,817,242; 6,117,344; 6,165,911; 6,168,845 B1; 6,190,929 B1; and 6,228,294 B1. A typical thermal imprint lithographic process for forming nano-dimensioned features in a substrate surface is illustrated with reference to the schematic, cross-sectional views of FIGS. 1(A)-1(D).
Referring to FIG. 1(A), shown therein is a mold 10 including a main (or support) body 12 having upper and lower opposed surfaces, with a molding layer 14 formed on the lower opposed surface. As illustrated, molding layer 14 includes a plurality of features 16 having a desired shape or surface contour. A substrate 18 carrying a thin film layer 20 on an upper surface thereof is positioned below, and in facing relation to the molding layer 14. Thin film layer 20, e.g., a thin film of polymethylmethacrylate (PMMA), can be formed on the substrate/workpiece surface by any appropriate technique, e.g., by a spin coating technique.
FIG. 1(B), is illustrative of a compressive molding step, where mold 10 is pressed into the thin film layer 20 in the direction shown by arrow 22, so as to form depressed, i.e., compressed, regions 24. In the illustration, features 16 of the molding layer 14 are not pressed all of the way into the thin film layer 20 and thus do not contact the surface of the underlying substrate 18. However, the top surface portions 24a of thin film 20 can contact depressed surface portions 16a of molding layer 14. As a consequence, the top surface portions 24a substantially conform to the shape of the depressed surface portions 16a, as for example, a flat surface. When contact between the depressed surface portions 16a of molding layer 14 and thin film layer 20 occurs, further movement of the molding layer 14 into the thin film layer 20 stops, due to the increase in contact area, leading to a decrease in compressive pressure when the compressive force is constant.
FIG. 1(C) shows the cross-sectional surface contour of the thin film layer 20 following removal of mold 10. The molded, or imprinted, thin film layer 20 includes a plurality of recesses formed at compressed regions 24 which generally conform to the shape or surface contour of features 16 of the molding layer 14. Referring to FIG. 1(D), in a next step, the surface-molded workpiece is subjected to processing to remove the compressed portions 24 of thin film 20 to selectively expose portions 28 of the underlying substrate 18 separated by raised features 26. Selective removal of the compressed portions 24 may be accomplished by any appropriate process, e.g., by reactive ion etching (RIE) or wet chemical etching.
The above-described imprint lithographic processing is capable of providing sub-micron-dimensioned features, as by employing a mold 10 provided with features 16 comprising pillars, holes, trenches, etc., patterned by means of e-beam lithography, RIE, or other appropriate patterning method. Typical depths of features 16 range from about 5 to about 200 nm, depending upon the desired lateral dimension. The material of the molding layer 14 is typically selected to be hard relative to the thin film layer 20, the latter typically comprising a thermoplastic material which is softened when heated. Suitable materials for use as the molding layer 14 include metals, dielectrics, semiconductors, ceramics, and composite materials. Suitable materials for use as thin film layer 20 include thermoplastic polymers which can be heated to above their glass temperature, Tg, such that the material exhibits low viscosity and enhanced flow.
Nanoimprint lithographic techniques effect the possibility of a low-cost, mass manufacturing technology for fabrication of sub-100 nm structures, features, etc. The problems, however, associated with this technique include, for example, non-uniform replication and sticking of the thermoplastic polymer materials to the molding layer 14. The uniformity and sticking difficulties tend to be more pronounced when the mold or stamper is applied to a large-area substrate, e.g., as in the formation of servo patterns in 95 mm diameter disks used in hard disk drives and is increasingly problematic as the feature size is reduced. Poor mold release, i.e. sticking, causes peeling or otherwise damage to the imprinted layer resulting in the degradation of dimensional integrity of the imprinted pattern or feature.
Stampers have been employed in various recording arts. For example, U.S. Pat. No. 4,252,848 to Datta et al. discloses methods of applying perfluorinated polymer films to a substrate. In Example 7, Datta et al. disclose a coated metal stamper for the production of vinyl polymer disks, i.e. conventional record albums for recording music. U.S. Pat. No. 4,482,511 to Komatsubara describes a method for manufacturing a stamper including depositing a low surface energy film on a stamp-forming-master. A stamper is then made from the master. U.S. Pat. No. 5,330,880 to Horigome et al. relates to a process for producing optical disks including treating the surface with a releasing agent to facilitate the process.
In view of the above, there exists a need for improved methodology and means for performing imprint lithography which eliminate, or at least substantially reduce, the disadvantageous degradation of imprint quality associated with the use of a stamper for sub-micron lithography. More specifically, there exists a need for an improved means and methodology for sub-micron imprinting of a pattern, e.g., a servo pattern, in a surface of a resist or other type relatively soft material on the surface of a substrate for information storage and retrieval medium, e.g., a hard disk magnetic recording medium.